I tested some offices yesterday and today. Electricians installed new ring ccts and replaced the ccu last week.
They used to be 2 houses so now they got suspended ceilings with 1200mm x 600mm and 600mm x 600mm fluorescent fittings.
Yesterday I was there and within 30min from switching the lights on, burning smell and smoke came out from one of the fittings. I switched the power off and found that the capacitor from a 1200mm x 600mm melted. That fitting disconnected straight away.
This morning I went back and the same scenario. In the other side of the building another capacitor melted and smoke was everywhere.
They fed from different ccts. The building is empty for a month now and it was very cold inside, about 0 degrees C. The fittings are about 5 years old but in good condition. The electricians used the lights for 3 days, last week without any problems. I check the voltage and it was 241V.
Test results were normal. Capacitors are 16microfarad.

Any suggestion for the reason caused capacitors to melt.
My supervisor believe is a coincidence but I am not agree with him.

there is not condesation.
the lighting ccts are exsisting, so the electricians just move them from the old ccu to new ccu. The capacitors were on different ccts and different mcbs. I ll check the neutrals.

I've never heard of a loose neutral causing a pf capacitor in a fluorescent fitting to burn out and I'm ancient and have connected thousands. I must have made lots of loose neutral but have never once seen a overheated capacitor in one of my installed fittings. Perhaps I've been lucky!

I suspect triplen harmonics causing sympathetic 3rd harmonic voltages on a high impedance supply. When 3rd order voltages meet the pfc caps they cause currents to flow in them which are much greater than they would be with just the 50Hz fundamental, hence the caps cook.

I had a spate of pf capacitors burn and smoke in fluorescent fittings recently. One lot was in Tam**** fittings, the other in Crom****. All were fitted at about the same time and all started failing around the same time.
Unless there is severe or regular over voltage I put it down to age.

In the past Investigated a couple of fluorescents caught fire, the cause was lost neutral and whole place was supplied with three phase only. Nearly equal balance saved the day but not the fittings.
Below is only to stimulate your thoughts, rule out unbalanced mains voltages.

I'd go with either a bad batch of capacitors although unlikely given they've all gone so close together or a neutral problem.

My money is on a neutral problem. Its not a neutral problem within the installation - it will be a joint that has developed a high resistance out on the network. The network being 3 phase - what happens when you loose the neutral with an unbalanced load (which the network is) the voltage goes off all over the place and you can end up with 400v accross the line and neutral.

You may have tested and got 214v but that is a sign that there is a problem already - should be very near 240 - as far as I know they haven't started changeing distribution voltage of regional electricity companies down to 230v yet although we use the 230v as nominal for design purposses. If you can measure it on a gegular basis or use a voltage monitor to do so then you will find its going up and down particularly as the distribuiton network loads change. This can often happen when off peak heating is on as it changes the balance completely.

If you can find evidence of these voltage changes then you'll need to report it to the supply authority as they will need to find the offending joint and repair - also you can claim for damage to equipment caused by their network problem.